There have been widely used image forming apparatuses which have an image bearing component, an exposing section, a developing section, a transferring section, and a fixing section, and in which the exposing section forms an electrostatic image on the image bearing component; the developing section develops the electrostatic latent image into a toner image with the use of developer which contains toner and carrier; the transferring section transfers the toner image onto a sheet of recording medium; and the fixing section fixes the toner image to a sheet of recording medium by the application of heat and pressure. As an image forming operation continues, toner consumption continues. Consequently, the developer in the developing section reduces in TD ratio (toner density: weight ratio of toner in developer). As the developer reduces in TD ratio, the developer supplying section supplies the developing section with replenishment developer which contains toner.
Japanese Laid-open Patent Application No. 2005-62848 discloses an image forming apparatus, the developing section of which is provided with an inductance sensor for detecting the TD ratio. In the case of this image forming apparatus, in order to maintain the TD ratio of the developer at a preset level (preset value), the amount by which the replenishment developer is supplied to the developing section from a replenishment developer container is adjusted according to the output of the induction sensor. As the TD ratio of the developer falls below the preset level, the control section of the apparatus determines that the replenishment developer container has become empty, interrupts the ongoing image forming operation, and displays a message which suggests the need for replacement of the replenishment developer container, on the control panel.
The density of an image which an image forming apparatus outputs corresponds to the amount (weight per unit area) by which toner is adhered to the electrostatic latent image to develop the electrostatic image. The amount by which toner is adhered to an electrostatic image is affected by the state of the image bearing component after the charging of the image bearing component by the charging section, state of the image bearing component after the exposure of the image bearing component by the exposing section, and state of the electrostatic image after the development of the electrostatic image by the developing section.
Japanese Laid-open Patent Application No. 2005-345961 discloses an image forming apparatus equipped with an optical sensor for detecting the amount (per unit area) of toner in the toner image formed on the image bearing component. In the case of this image forming apparatus, an image forming operation is periodically interrupted, and multiple test images (toner images) which are different in the amount of output of the exposing section, are formed. Then, the test images are detected by an optical sensor. Then, the exposing section is adjusted in the amount of output, based on the results of the detection, in order to ensure that the image forming apparatus outputs images which are proper in the amount of toner per unit area.
As the developer in the developing section reduces in the TD ratio, the developing section increases the amount of charge it gives to toner. Consequently, it reduces in the amount by which it adheres toner to an electrostatic image, and therefore, is likely to cause the image forming apparatus to reduce in image density.
Thus, Japanese Laid-open Patent Application No. 2005-62848 proposes an image forming apparatus designed so that as it determines, based on the output of its induction sensor, that the developer has reduced in TD ratio, its replenishment developer supplying section replenishes the developing section with replenishment developer. However, it sometimes occurs that even though the operation for replenishing the developing section with replenishment developer is carried out by the replenishment developer supplying section, the developer in the developing section continues to reduce in TD ratio.
For example, as a replenishment developer container becomes almost empty, even if the replenishment developer supplying section carries out the operation for replenishing the developing section with replenishment developer, the developing section is not supplied with a sufficient amount of toner, and therefore, the developer in the developing section continues to reduce in TD ratio. If the replenishment developer container becomes empty, the developing section is not replenished with toner no matter how long the operation for replenishing the developing section with replenishment developer is continued, and therefore, the developer in the developing section continues to reduce in TD ratio. Consequently, the image forming apparatus continues to reduce in image density, outputting therefore several tens of prints which are inferior in image quality.